In traditional RFID reader systems, each reader consists of one transmitter and one receiver. In a typical configuration, the transmitter and the receiver are attached to a multiplexer. This configuration allows for the transmitter and receiver to be connected to independently to selected antenna elements (e.g., “switched bistatic” or “multistatic” configurations). Alternatively, in a “switched monostatic” configuration a circulator or directional coupler is used to separate transmit and receive paths from one of a number of switched antenna elements. The signals received from RFID tags are coupled from the tag, through the air, to a single receiving antenna element. However, noise is also present at the receiving element and reduces the sensitivity of the reader's receiver.
There are four main sources of noise present in the reader's receiver. A first source of noise is generated by active elements in the receiver. Examples include, but are not limited to, receiver local oscillator phase noise, mixer resistive (Johnson) noise, and 1/f noise present in the receiver's baseband chain.
A second source of noise is produced in the RFID reader's transmitter section and coupled to the receiver by virtue of the finite transmit-receive isolation dependent on the chosen antenna configuration (e.g., monostatic or bistatic). This noise consists of the reader's transmitted composite noise, consisting of local oscillator phase noise, modulation DAC quantization noise, transmit modulator noise, and the noise contributions of the amplifiers needed to generate the final transmitted power.
A third source of noise is backscatter noise from external source. The backscatter noise is caused by time-varying reflections in the environment. This noise is often produced by fluorescent lighting, rotating machinery, etc, and is effectively environment dependent in nature and composition. In some instances, this noise occurs at multiples of the AC power line frequency (in the case of noise backscattered by rotating machinery) or at the high frequencies used for fluorescent lighting ballasts.
A fourth source of noise is jammer noise that is received in the receiver antenna from external transmitters, especially other RFID reader transmitters or other users of the RFID band. This noise is often the dominant contributor to the observed signal-to-noise ratio in the reader's receiver path.